The tonality of the song is either C major or D minor and F major, not as it is written in a few musical scores. The range of notes is the same as C major. But the harmonic center of the song is the D-minor chord. Such melodies are said to be in so-called "Dorian scale". This is a church key. So named because there are many old hymns in this key. But there are some folk songs (eg the Irish folk music) and modern pieces (eg, Mad World by Michael Andrews) still using this mode.